This invention relates to liquid crystalline materials and, particularly, to uses of the combination of the twisted nematic structure in nematic liquid crystalline materials in contact with extrinsically optically active materials. More particularly, this invention relates to the use of the discovery that the optically active materials display circular dichroism when in contact with the twisted nematic structure in nematic liquid crystalline materials.
Liquid crystalline substances exhibit physical characteristics, some of which are typically associated with liquids and others which are typically unique to solid crystals. The name "liquid crystals" has become generic to substances exhibiting these dual properties. Liquid crystals are known to appear in three different forms: the smectic, nematic, and cholesteric forms. These structural forms are sometimes referred to as mesophases thereby indicating that they are states of matter intermediate between the liquid and crystalline states. The three mesophase forms of liquid crystals mentioned above are characterized by different physical structures wherein the molecules of the compound are arranged in a manner which is unique to each of the three mesomorphic structures. Each of these three structures is well known in the liquid crystal art.
The twisted nematic structure is well known in the art. When a nematic liquid crystalline material is sandwiched between substrates treated for unidirectional homogeneous alignment, the nematic is externally perturbed into a helical structure when the rubbed surfaces are placed at some angle (other than 0.degree.) with respect to one another. See, for example, M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971), "Voltage-Dependent Optical Activity of A Twisted Nematic Liquid Crystal".
Twisted nematic cells have been employed in display devices, particularly where electro-optic effects are modulated by the application of an electrical field across layers of nematic liquid crystalline meterial in the twisted nematic structure. See, for example, U.S. Pat. No. 3,731,986 to Fergason and 3,781,085 to Leibowitz.
The molecules of the nematic in the twisted structure of nematic liquid crystals can be thought of as arranged in very thin layers with the long axes of the molecules parallel to each other and to the local optic axes of the layers. The direction of the long axes of the molecules in each layer is displaced slightly from the corresponding direction in adjacent layers. This displacement is cumulative over successive layers so that overall displacement traces out a helical path. A comprehensive description of the optical properties of the twisted nematic structure of nematic liquid crystals is given in "Optics in Smoothly Varying Anisotropic Planar Structures: Application to Liquid-Crystal Twist Cells", D. W. Berreman, 63, J. Opt. Soc. Am., 1374 (1973).
Nematic liquid crystals in the twisted structure rotate the plane of polarization of light passing therethrough an amount between 0.degree. and 90.degree. equal to the angular difference between the alignment directions of the substrates. The intrinsic sense of the helical twist can be either right-handed or left-handed.
Extremely large extrinsic circular dichroism has been observed within the electronic transitions of achiral (optically inactive) solutes dissolved in cholesteric mesophases as reported in recently issued U.S. Pat. No. 3,780,304 to F. D. Saeva, et al and in the following articles by F. D. Saeva et al appearing in the Journal of the American Chemical Society (JACS): "Cholesteric Liquid-Crystal-Induced Circular Dichroism (LCICD) of Achiral Solutes. A Novel Spectroscopic Technique", Vol. 94, JACS, page 5135 (1972); "Cholesteric Liquid-Crystal-Induced Circular Dichroism (LCICD). V. Some Mechanistic Aspects", Vol. 95, JACS, page 7675 (1973); "Cholesteric Liquid-Crystal-Induced Circular Dichroism (LCICD). VI. LCICD Behavior of Benzene and Some of its Mono- and Disubstituted Derivatives", Vol. 95, JACS, page 7660 (1973); and "Cholesteric Liquid-Crystal-Induced Circular Dichroism (LCICD). VII. LCID of Achiral Solutes in Lyotropic Cholesteric Mesophases", Vol. 95, JACS, page 7882 (1973).
To our knowledge, circular dichroism has not been previously reported as induced in extrinsically optically active materials in contact with the twisted structure of nematic liquid crystalline materials.
It has heretofore been thought by those working in the art as evidenced by the above articles that two mechanisms contributed independently to the existence of Liquid Crystal Induced Circular Dichroism in dissolved materials: (1) helical organization of solute, and (2) the exposure of solute to a helical organization of liquid crystal molecules. My work prior to the present invention indicated that mechanism (2) could be the sole operative mechanism. Shortly after the invention described in this Applicaton, independent data was reported which confirmed that mechanism (1) was not required for the observation of extrinsic LCICD within solutes in the cholesteric mesophase. That is, the solute molecules need not be ordered into helical organization by the mesophase in order to exhibit liquid crystal induced circular dichroism. The data reported in "The Optical Activity of Achiral Molecules in a Cholesteric Solvent", J.C.S. Chem. Comm., page 712, 1973.